Question { 30070 }

Why we connect the capacitor in Delta?if we conect in Star
what will happen?

Answer

Wye connected capacitors connected to motors are not
appropriate because they are ungrounded and thus present a
potential difference on each phase of the capacitor which
contributes to the over-all deterioration of the capacitor.
As can be seen in fig.1 below, the failure of any one cell
of a Wye connected capacitor presents an unbalanced load to
the motor. This will effectively destroy the motor due to
unbalanced voltage, temperatures.
Example: If you have a 100HP motor and an unbalance of 5%,
this will cause a power loss of 25%. (100HP – 25% = 75HP).
If the motor was loaded to 85%, the motor now becomes
overloaded. The motor would run 113.5% overloaded, therefore
overheated and will eventually burn out.
Unprotected Wye connected capacitors should not be used for
Power Factor correction. The unbalance protection required
for this type of application is more costly than the
capacitors themselves and therefore not included with Wye
connected capacitors.
Wye connected capacitors do not circulate harmonics and thus
are much more vulnerable to failure due to harmonics.

Fig.1 - The total capacitance for the example above with a
failed cell would be 5kVAR
Delta connected three phase capacitors are more friendly to
the electrical environment. They not only provides
capacitance to each phase but also provides no voltage
unbalances (harmonics).
If a capacitor cell should fail for any reason in one phase,
we are going from a closed delta to an open delta
configuration and in this instance the voltages on each
phase stays the same (see fig.2 below). Failure of a
capacitor cell would only mean a drop in the kVAR on the
circuit. In other words, a 30kVAR delta connected three
phase capacitor with a failed cell would provide 20kVAr to
the electrical system instead of 30kVAR.
Delta connected capacitors unlike the Wye connected
capacitors are able to circulate harmonics currents within
the delta circuit. By doing so it reduces the harmonics on
the electrical system.


Fig.2 - The total capacitance for the example above with a
failed cell would be 20kVAR